Changing and testing pH

After improving porosity, changing pH is the most common reason for altering soils. Raising and lowering pH both are necessary at times, depending, of course, on the pH with which you’re starting.

l Reducing acidity. Liming is the practice of applying an agent to reduce soil acidity (raise pH) and make soils more favorable for plant growth. The amount of lime you must add depends on the degree of soil acidity, the buffering capacity of the soil, the desired pH, and the quality and type of lime you use.

Liming materials. The most widely used liming materials for turfgrass areas consist of carbonates of calcium or magnesium. These include ground, pelletized and flowable limestone. Of these three, ground limestone is the type used most widely. Crystalline calcium carbonate (CaCO3), one type of ground limestone, is termed calcitic limestone. Dolomitic limestone, another ground-limestone product, comes from ground rock containing calcium-magnesium carbonate (CaMg[CO3]2) and has a higher neutralizing value that calcitic limestone. Dolomitic limestone not only lowers pH but also can supply magnesium in soils that are deficient. Although ground limestone is the most inexpensive source, it is dusty and not as easy to spread as the pelletized form.

Pelletized limestone is ground limestone (either calcitic or dolomitic) that has been aggregated into larger particles to facilitate spreading and reduce dust. The pellets quickly disintegrate when wet.

Flowable limestone is available for use on turf when you need to use a liquid application. Although liquid applications are dust-free and uniform, you only can apply relatively small amounts at one time and lime spray suspensions may be abrasive to sprayer parts.

Hydrated (slaked) lime [calcium hydroxide, Ca(OH)2] and burned lime (quicklime—calcium oxide, CaO) provide a rapid pH change but can be phytotoxic. These products are corrosive and difficult to handle.

As you might expect, sources of limestone vary in quality and effectiveness. Even two pelletized limestones made by different companies may vary in their ability to neutralize soil. To get the best bargain when purchasing lime products, look for quality, not just the lowest price. Two main factors govern the quality of a liming material: purity and fineness.

Purity. Most lime recommendations assume you will use liming materials that have the same neutralizing potential as pure calcium carbonate. In other words, if your soil-test report recommends that you apply 50 pounds of limestone per 1,000 square feet, it assumes you will use a lime source that will raise soil pH to the same extent as 50 pounds of pure calcium carbonate at the same rate. A liming material with the same neutralizing potential as pure calcium carbonate has a calcium carbonate equivalent (CCE) of 100 percent.

You should adjust the recommended rate of any liming material with a CCE of less than or greater than 100 percent so that you apply the right amount of material to raise your soil pH to the target level . Generally, because of impurities, such as clay, neutralizing value of most agricultural limestones is 90 to 98 percent. Most states require that agricultural liming materials state their CCE on the label.

Fineness. Any effective liming material should be finely ground. This is important because the rate at which limestone raises pH increases with the fineness of the particles. Plus, limestone affects only the small volume of soil surrounding each limestone particle. A given volume of limestone contains more particles if it is finely ground and thus affects more soil than coarser limestone. Many states govern the sizes of limestone particles in pelletized lime and agricultural ground limestone. Manufacturers usually print the actual range of particle sizes on the label. However, you will generally find little advantage in using material much finer than these minimum standards.

How and when to apply limestone. Lime will neutralize soil acidity and benefit turf growth faster if you incorporate it directly into the soil. You can incorporate lime by spreading a layer on the soil surface following a rough grading, then mixing the lime 4 to 6 inches into the soil with rotary tilling equipment. Not only does this practice distribute the lime throughout the entire root zone, you can apply much more in a single application than with a surface application. Often, you can supply the entire lime requirement in a single application during establishment, whereas several surface applications may be necessary on established turf or landscape beds.

A means of incorporating lime in established turf is through core aeration. If your soil-test report indicates that an area about to undergo renovation requires liming, apply the recommended amount of lime (along with any needed phosphorus and potassium) after herbicide treatment and thatch removal, and just before or just after aeration. As you aerate and drag the area, some of the lime/soil mix will fall into the aeration holes and some will remain on the soil surface. The more vigorous the aeration treatment the better the lime will mix with the soil.

Established turfgrass areas should not receive more than 25 to 50 pounds of limestone per 1,000 square feet in any single surface application. If you use hydrated or burned lime, apply no more that 25 pounds per 1,000 square feet in a single application. The main reason for this is to ensure that a layer of excess residue does not remain on or near the surface after watering or, in the case of hydrated or burned lime, that plant injury does not occur. If a soil requires more limestone than you can apply at one time, use semi-annual applications until you meet the requirement.

Ground limestone sometimes is difficult to spread with conventional spreaders. However, pelletized limestone spreads easily with conventional drop or spinner spreaders. For large areas, commercial spreader trucks are available for custom spreading. You can apply ground limestone anytime during the year, but it is most effective in the fall or winter because rain, snow and frost heaving help work limestone into the soil.

l Lowering soil pH—acidification. Soils often need acidification in semi-arid and arid regions or when you’ve applied excess lime. Plus, golf-course superintendents sometimes apply acidifying materials to their greens as a means of managing certain diseases. They accomplish this by applying ammonium-containing fertilizers such as ammonium sulfate or elemental sulfur, or by injecting sulfuric acid into their irrigation systems.

Ammonium-containing fertilizers are effective for lowering soil pH when you need only slight acidification over an extended period. In the northeastern United States, some golf-course superintendents use ammonium sulfate to lower the pH of putting greens affected by take-all patch and summer patch diseases. While this practice is effective in some cases, take care to avoid foliar burn and over-stimulation of turf with nitrogen. To avoid burning, make the applications during cool weather (spring and fall) at low rates. When using this approach for disease management, you should monitor soil-pH levels frequently to avoid nutrition and thatch problems caused by low pH.

If you require greater and more rapid acidification, you can use high-sulfur-content products. When you apply sulfur to soil, soil-borne bacteria convert it to sulfuric acid, thereby lowering soil pH. Powdered elemental sulfur typically is yellow and fairly pure (greater than 90 percent sulfur). As with lime, sulfur is more effective in a finely ground state. Several sulfur products are available in powder form but, as such, are dusty and not easy to apply with spreaders. You also can obtain sulfur in pelletized form (90 percent powdered sulfur and 10 percent bentonite clay). This is easy to spread with conventional fertilizer spreaders and quickly breaks down into the powdered form when moist. If you want to apply sulfur as a liquid, flowable forms also are available.

The best time to apply sulfur is before establishment. By applying sulfur directly to the soil surface, and then tilling it into the soil, sulfur will be in direct contact with soil microbes and distributed throughout the entire root zone. Incorporating sulfur before planting also allows you to use greater amounts than possible with surface applications on established turf.

Generally, sandy soils require smaller amounts of sulfur to lower pH than mineral soils. For example, lowering the pH of a 6-inch-deep layer of sandy soil from 8.0 to 6.5 requires 27.5 pounds of sulfur per 1,000 square feet. However, a clay soil needs 45.9 pounds of sulfur per 1,000 square feet for the same adjustment.

Established turf generally requires frequent applications of sulfur at relatively low rates to lower pH. On putting greens, applications normally are around 0.5 pound sulfur per 1,000 square feet and should not exceed about 2.3 pounds per 1,000 square feet per year. You can double these rates on high-cut turf if you apply the product in cool weather. Remember, excessive sulfur can injure turf, especially in hot and humid weather.

To determine if your sulfur applications are having the desired effect on pH, monitor your soil with laboratory tests. Make sure that you test the surface soil (upper 0.5 to 1 inch) separately because most of the sulfur you apply to established turf will remain and react near the soil surface. This possibly can create highly acidic conditions in the top 0.5 to 1 inch of the soil.

In recent years, some golf courses in the southwestern United States have used sulfuric-acid irrigation-system injections to acidify soil. At least one system uses pH electrodes and a computer to maintain water pH at a constant 6.5. If the pH falls outside the operating range, the system automatically shuts down. With innovations such as these, acidification of soils with acid injection undoubtedly will become more common in the near future.

pH testing

You can determine soil pH with one of several types of soil tests. However, not all soil tests provide accurate information about how much lime or acidifier you should apply. Test kits using dyes, pH pens or pH paper determine pH rapidly in the field. The least accurate means of determining soil pH is with pH paper, but it can be useful in obtaining an approximate value. While each of these tests can provide a fair indication of soil pH and tell you if you need lime, they do not provide accurate information on how much lime you should apply. These figures are only approximate—consult a soil lab before undertaking pH modifications.

Commercial and university testing labs accurately determine pH values for soils over a range of pH values. They also provide meaningful lime recommendations for acid soils. They base their lime recommendation on a lime-requirement test that tells you how much lime is necessary to bring the soil to an optimum pH. The lime requirement is based on the buffering capacity of a soil, or buffer pH. Regarding pH amendments, buffer pH is more important than active pH.

Each lab bases its lime recommendations on what they consider to be optimal pH for the turf or ornamentals you’re growing there. Before submitting your soil samples, realize that differences exist among labs regarding what they consider to be the optimum pH ranges for turfgrasses and ornamentals. This is why lime recommendations vary from one lab to another. The best way to deal with this problem is to choose a lab that provides recommendations that make sense to you and then stick with that lab for future testing to maintain consistency.